#include <iostream>
using namespace std;
int main()
{
	int k,i;
	int t=0,p=1,total=0;
	cin>>k;
	for (i=1;i<=k;i++)
	{
		if (t==p) 
		{
			t=0;
			p=p+1;
		}
		total = total + p;
		t++;
	}
	cout<<total<<endl;
	return 0;
}
